In the manufacture of microelectronic hardware and other products, pneumatic dispensing apparatus are used to dispense small amounts or droplets of a highly viscous material in a non-contact manner onto a substrate or workpiece. Exemplary highly viscous materials include, but are not limited to, solder flux, solder paste, adhesives, solder mask, thermal compounds, oil, encapsulants, potting compounds, inks, and silicones. Generally, such highly viscous materials cannot easily flow under their own weight at room temperature.
Conventional pneumatic non-contact dispensing apparatus for viscous materials include an air-operated valve element reciprocated for selectively engaging a valve seat surrounding a discharge passageway. In a process commonly referred to as jetting, droplets are dispensed by retracting the needle from contact with the valve seat, which allows an amount of the viscous material to flow under pressure from a filled fluid chamber through a gap separating the needle from the valve seat and into the discharge passageway. The needle is then moved rapidly toward the valve seat to close the dispensing apparatus, which causes the amount of viscous material to be forced through the discharge passageway and a comparable amount of the viscous material to be ejected from a discharge orifice of the discharge passageway. The small amount of ejected viscous material is propelled as a droplet toward a workpiece, which is spaced from the discharge outlet.
Valve seat replacement and cleaning in conventional non-contact dispensing apparatus is a time consuming and painstaking process as the internal surfaces of the dispensing apparatus are difficult to access with cleaning tools. Generally, the valve seat is integral with the fluid chamber and, as a result, is non-removable, which restricts access to the fluid chamber and creates a circular right angle corner at their juncture that is difficult to clean. In addition, the valve seat may include guide fingers or vanes that guide the needle so that a needle tip makes a reproducible fluid seal with the valve seat tolerant of minor misalignments. However, the guide vanes define right angle corners that are difficult to adequately clean effectively in a short time.
Disassembling and reassembling conventional non-contact dispensing apparatus is a difficult process that involves numerous tools. In addition, gauges are required to establish accurate spatial relationships between components during reassembly. As a result of the complexity, disassembly and reassembly are slowed and may take as long as forty-five (45) minutes to complete, even for technicians skilled in the assembly procedure.
In certain conventional dispensing apparatus, the valve seat in the fluid chamber and the tip of the needle constitute a matched pair carefully lapped to have corresponding dimensional attributes. Any attempt to replace the valve seat to, for example, change the diameter of the discharge passageway often results in leakage because the needle tip and the new valve seat are not a matched pair and, therefore, cannot provide an adequate seal. In such conventional dispensing apparatus, therefore, the diameter of the dispensing orifice may be changed only by replacing the existing needle and valve seat with a needle having a needle tip matched during manufacture with the valve seat.
Another problem encountered in conventional pneumatic non-contact dispensing apparatus is noise. The dispensing apparatus is opened and closed by switching a solenoid valve to provide and remove pressurized air from an air piston cavity. The pressurized air acts on an air piston that reciprocates the needle. To close the dispensing apparatus, the solenoid valve is switched to exhaust air pressure from the air piston cavity to the ambient environment through an exhaust passageway. The rapid flow of air through the exhaust passageway causes sound audible to bystanders. A conventional silencer or muffler may be used to reduce noise at valve exhaust ports. However, the exhaust port of the solenoid valve must be accessible to permit attachment, typically by a threaded connection, of the muffler.
It would be desirable, therefore, to provide a dispensing apparatus that overcomes these and other deficiencies of conventional dispensing apparatus for viscous materials, as described herein.